Project Summary/Abstract C?H bond transformations lie at the heart of numerous metabolic pathways throughout the biosphere. Oxidoreductase enzymes manipulate strong X?H bonds (X = C, N, O) with limited free energy expenditure through a process known as multiple-site concerted proton-electron transfer (MS-CPET) which underlies photosynthesis, respiration, and complex biomolecule synthesis. In many of these reactions, the proton transfer coordinate is guided by pre-aligned hydrogen bonding interactions, which are absent with C?H bonds. Consequently, the mechanisms of many CH-CH oxidoreductase enzymatic reactions are not known despite the large number that rely on distant electron transfer cofactors. Therefore, deriving principles behind MS-CPET involving C?H bonds would be immensely informative to unveiling how a significant amount of enzymes function in the biosphere. In this proposal, we seek to understand how the proton transfer coordinate governs C?H bond reactivity through MS-CPET using molecular models. Specifically, we will probe how key aspects such as proton transfer pre-alignment and proton tunneling distance affect C?H bond cleavage through structural variation. We will also derive essential thermochemical principles for reductive C?H bond formation in a stable radical system to develop kinetic free energy relationships. Finally, we will build upon these kinetic and thermochemical models to assess the reductive hydrogenation mechanism of aromatic substrates central to anaerobic microbe metabolism. Our goal is to develop a mechanistic understanding of MS-CPET with C-H bonds in our model systems to illuminate unknown CH-CH oxidoreductase reactivity. This research will help elucidate how enzymes perform difficult C?H bond transformations and guide synthetic chemists towards new approaches for manipulating strong bonds. My postdoctoral training in the Mayer group will expand my research skillset through learning mechanistic and kinetic studies as well as allowing me to hone my mentorship, writing, and presenting skills. Importantly, this training will teach me new ways to think and approach scientific problems allowing me to expand the scope of research I can address in my future independent career. Yale University fosters an ideal environment to train me with its exceptional facilities, seminar and teaching opportunities, and prominent faculty who are experts in synthetic organic, theoretical, and biological chemistry. I believe my training will sufficiently equip me to be a leading independent researcher and teacher at an academic institution in the future.